Neopterin (2-amino-6-(1,2,3-trihydroxypropyl)-1H-pteridin-4-one) is a marker of cellular immune system activation. Using SERS, we are able to detect neopterin at low concentration in blood plasma and bacteria-infected cerebrospinal fluid. In these studies, three meningitis pathogens, namely, Neisseria meningitidis, Haemophilus influenzae, and Streptococcus pneumoniae were detected and identified simultaneously using a label-free SERS method.